Magnetic recording media are widely used in the fields of recording tapes, video tapes, tapes for computers and floppy disks. The magnetic recording medium is basically composed of such a structure including a magnetic layer composed of ferromagnetic fine particles dispersed in a binder which is laminated onto a non-magnetic support.
Generally speaking, the magnetic recording media are demanded to have high performance capability with regard to characteristics such as electromagnetic characteristics, running durability and running performance. Particularly, it is required that video tapes have high video output and are excellent in the reproducibility of the original picture, particularly electromagnetic characteristics with the spread of contemporary eight mm video, high band eight mm video and S-VHS videotape recorders.
Many methods for improving the electromagnetic characteristics of the magnetic recording media have been proposed. Among them, methods for improving the characteristics of ferromagnetic fine particles, which are magnetic recording substances, have been shown to be capable of directly improving electromagnetic characteristics and are effective. For instance, ferromagnetic powders have been finely divided into finer powders in order to enhance the capability of conducting high-density recording. Further, the materials of ferromagnetic powders have been changed from iron oxide to iron oxide modified with different metals such as cobalt. Further, ferromagnetic metals such as iron, nickel and cobalt or alloys thereof have been used.
Magnetic recording media having good electromagnetic characteristics can be essentially obtained by using such improved ferromagnetic fine powder. However, as a practical matter, it is difficult to produce magnetic recording media having improved electromagnetic characteristics equivalent to a degree which is proportional to the improvement in ferromagnetic fine powders per se. This is attributable to the fact that the dispersibility of ferromagnetic powders has a tendency to decrease in binders along with any reduction in the particle size of the ferromagnetic powders. Further, the dispersibility of ferromagnetic fine powders have characteristics such that the dispersibility is apt to be more greatly lowered in the order of .gamma.-iron oxide, cobalt-deposited .gamma.-iron oxide and fine powders of ferromagnetic metals. As a result, there is a possibility that the dispersive condition of the ferromagnetic fine powders may become poorer conversely to improvements provided in the ferromagnetic fine powders per se. Accordingly, the excellent electromagnetic characteristics of the ferromagnetic fine powders cannot be sufficiently exhibited and as a result, magnetic recording media having improved electromagnetic characteristics which are proportional to the degree of improvement provided in the ferromagnetic fine powders per se cannot be obtained.
Many attempts to improve the dispersibility of ferromagnetic fine powders have been made. For example, binders having a polar group such as --SO.sub.3 M, --OSO.sub.3 M, --PO.sub.3 M.sub.2, --OPO.sub.3 M.sub.2, --COOM, --NR.sub.2 or --N.sup..sym. R.sub.3 X.sup..crclbar., or binders having said polar group and an epoxy ring, as the improvement of binders are disclosed in JP-B-58-41565 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-57-44227 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-59-30235, JP-A-60-238306, JP-A-60-238309, JP-A-60-238371 and JP-A-61-172213. These binders have high adsorptivity to ferromagnetic fine powders and good dispersibility in comparison with conventional binders. However, even when these binders are used, the finer the particle size provided in the ferromagnetic fine powder for purposes of high density recording, the more difficult the dispersion thereof becomes, and, as a result, excellent electromagnetic characteristics of ferromagnetic fine powder cannot be sufficiently exhibited. Accordingly, a demand has arisen to develop a method for sufficiently exhibiting the electromagnetic performance of highly dispersible binders.
In response to this demand, one suggested method involves a kneading-dispersion carried out for a large number of hours during the preparation of magnetic coating solutions in order to improve the dispersion state of the ferromagnetic fine powders. However, there is a problem associated with the use of kneading in that the characteristics of ferromagnetic fine powders are deteriorated because considerable shearing forces are exerted on the ferromagnetic fine powders during kneading dispersion. Further, this method has a problem in regard to production efficiency, that is, manufacturing cost, since a large number of hours are required for the production of the magnetic recording media due to the prolonged kneading step.
In order to solve the above-mentioned problems, there have been proposed a method using surface-treated ferromagnetic fine powders obtained by treating the surfaces of the powders with a surface treating agent such as a silane coupling agent, and a method wherein dispersion is carried out by using dispersants such as fatty acids or phosphoric esters, and a method wherein a product obtained by adding a low-molecular epoxy resin to a system comprising a vinyl copolymer and a urethane prepolymer is used as a binder (see, e.g., JP-B-56-23210).
When the surfaces of ferromagnetic fine powders are treated by the aforesaid methods using the silane coupling agent or the dispersants, the affinity of the surfaces of ferromagnetic fine powders with the binders can be improved and the dispersion rate is increased. However, the ferromagnetic fine powders are not firmly bonded to the binders and dispersibility, including dispersion stability, is still insufficient.
When dispersion is carried out by using low-molecular epoxy resins or compounds having an epoxy group, there are disadvantages from the viewpoint of undesired reactions between --OH groups derived from epoxy groups during kneading and polyisocyanate compounds used as hardening agents. That is, when polyisocyanate compounds are used in combination with said epoxy resins or compounds having epoxy groups to improve the mechanical strength and running durability of magnetic layers the epoxy group is opened during kneading and dispersion in the preparation of magnetic coating solutions, and the formed --OH group reacts with the polyisocyanate compounds. As a result, the pot life of the magnetic coating solutions is greatly shortened.
There are known methods using ferromagnetic fine powders obtained by treating the surfaces of the powders with a surface treating agent such as a coupling agent having amino group, --OH group or epoxy group as a functional group capable of being bonded to the binders to solve the above-mentioned problem (see, JP-A-56-143533, JP-A-57-186302, JP-A-58-155517, JP-A-58-155703, JP-A-58-205929, JP-A-59-079433, JP-A-60-173721, JP-A-61-026935, JP-A-60-107731, JP-A-60-127526 and JP-A-1-173321).
On the other hand, when the particle size of ferromagnetic fine powders becomes extremely fine, the thickness of the binder coating the ferromagnetic powders becomes thinner and, as a consequence, elongation at break of the magnetic layers becomes small and very brittle magnetic layers are formed if the amount of the binder is not increased. When the magnetic layers are brittle in nature, a problem is encountered when tapes are repeatedly run in that the magnetic layers at the edge parts of the tapes are liable to be cracked, and the magnetic layer material at the cracked portions are broken and dropped out. As a result, drop out is increased. Alternatively, powder drops out from the tape edge part of the magnetic layer during the slitting stage of original tape production, and drop out is increased.
To solve the above-mentioned problems, there have been proposed a method wherein the amounts of the polyisocyanate compounds to be added are reduced, and a method wherein plasticizers are added. However, these methods are only effective in the inhibition of brittleness, yet increase the temperature dependence of the mechanical strength of the magnetic layers and cause a lowering in mechanical strength under high-temperature conditions in particular. Hence, durability is lowered by these methods also.
When the aforesaid coupling agents having a functional group are used together with the polyisocyanate compounds, the polyisocyanate compounds are reacted with the functional group of the coupling agent and further with the binders having --OH group to thereby bond them to each other so that the mechanical strength and heat resistance of the magnetic layers can be improved. However, since the functional group of the coupling agents is readily reacted with the polyisocyanate compounds in the magnetic coating composition, there is a serious disadvantage that the viscosity of the coating composition is greatly increased rendering virtually impossible the coating of the magnetic layers during the tape manufacturing process. Or, even when the coating can be conducted, the smoothness of the surfaces of the magnetic layers is greatly lowered and electromagnetic characteristics are lowered.
Accordingly, an effective solution has been highly demanded insofar as to develop a magnetic recording medium for high density recording, in which ferromagnetic fine powder having a very small particle size is highly dispersed, and which has very high electromagnetic characteristics and running durability.